In a conference call with reporters today, FDA's Sherri McGarry says the agency's tests of cantaloupes growing in the fields were negative. Same for some samples collected in the packing facility, although she noted they were collected after the equipment had been thoroughly cleaned.

Listeria bacteria were found on some of the fruit in cold storage and some equipment, but not in the water used to wash the cantaloupes. (Read the full inspection report here.)

But where did the bacteria come from? A farm truck used to haul cantaloupes to a cattle facility — increasing the risk of contamination via animal feces — is one possibility.

Also, Magary says, the packing facility has a floor that allows water to pool and contains equipment previously used on another crop — potatoes — that wasn't "easily cleanable and sanitized." And, the packing facility is open to the air.

She added that there is no reason to believe these practices are widespread in the industry.

Jensen Farms is currently not operating and has agreed to allow government inspectors to inspect it before reopening. It initiated a recall Sept. 14.

FDA Commissioner Margaret Hamburg says she hopes the listeria investigation will lead to better prevention of foodborne illnesses in the future - a mantra the agency has been repeating since the first reports came out last month.

As of yesterday, the outbreak has been blamed for at least 25 deaths and 123 illnesses in 26 states, the Centers for Disease Control and Prevention reports. While the casualties have slowed since September, they still may creep up, given the long incubation period of listeria, officials say.

Still, not everyone who was exposed got sick, and our own Allison Aubrey reported on why that is recently.

Stay tuned for more on the listeria outbreak tonight on All Things Considered.

Copyright 2013 NPR. To see more, visit http://www.npr.org/.

Transcript

ROBERT SIEGEL, HOST:

The Food and Drug Administration has pinpointed what it says are the likely causes of the recent Listeria outbreak. That outbreak has been blamed for at least 25 deaths and 123 illnesses. The FDA identified a series of problems at a Colorado cantaloupe farm.

And as NPR's Allison Aubrey reports, what investigators found inside the farm's packing facility offers important clues as to what went wrong.

ALLISON AUBREY, BYLINE: Investigators found low levels of Listeria contamination on some hard-to-clean equipment inside Jensen Farms packing facilities. And experts say the virulent strains of bacteria that made people sick likely found lots of places to hide and multiply. They also found water, pools of water on the floor of the facility.

Don Schaffner is a food science expert at Rutgers University.

DON SCHAFFNER: Water gets moved around in a food processing plant or a packing house all the time. With hoses, it gets sprayed. It's just - water is the vehicle that facilitates the movement of bacteria in many different environments.

AUBREY: If water was spreading the bacteria it could explain how it ended up on the fruit. But how did the bacteria get to the packing facility in the first place? Well, investigators learned that a truck at Jensen Farms used to haul melons had also made trips to a cattle operation, possibly introducing animal feces.

During a press conference today, FDA Commissioner Margaret Hamburg said Jensen Farms has agreed to correct all the problems identified during the investigation. And she says, going forward, prevention must be the cornerstone of food safety.

DR. MARGARET HAMBURG: The lessons from this outbreak also speak to the urgency we have in implementing the FDA's Food Safety Modernization Act, which gives FDA much-needed tools to build a new prevention-focused food safety system.

AUBREY: Administrators are in the process of writing new standards to codify best practices that all food producers should be following. But Erik Olson, who directs food programs for the Pew Charitable Trust, says he's concerned that proposed budget cuts may hamper the progress here.

ERIK OLSON: We are concerned that not only investigations of outbreaks but, and frankly even more importantly, preventing these kinds of outbreaks with new standards that are supposed to be put into place fairly soon, may not happen if these budget cuts go into effect.

AUBREY: The FDA says Jensen Farms is currently not operating and has agreed to allow government inspectors to inspect again before reopening.